Contact type linear image sensor (Contact Image Sensor; hereinafter, referred to as CIS) 12 utilized in an image reading apparatus to optically read an original image has a constitution in which plural number of sensor chips 12a are connected one after another along the horizontal scanning direction. In such a constitution, since it is difficult to precisely make the inter pixel distance of length Lb corresponding to one pixel (approximately 42.3 μm in the case of 600 dpi) at the connecting portion between sensor chips 12a, many contact type line sensors adopt interval La corresponding to approximately two pixels (approximately 84.6 μm in the case of 600 dpi) at the connecting portion, thus allowing one missing pixel K at said connecting portion.
When an output signal of such a contact type linear image sensor is utilized without paying any attention to the missing pixel, a step will be generated in a line image at the connecting portion having a missing pixel, or a stripe noise will be generated in a halftone dot having a periodicity, resulting in deterioration of image quality. Therefore, in order to make the stripes less visible, there is often used a filter which interpolates a missing pixel by employing, as reference pixels, the peripheral pixels along the one dimensional direction including a missing pixel (for example, the horizontal scanning direction (the line direction of a contact type linear image sensor)).
Such interpolation is effective for a line image and a natural image; however, unnatural stripes will be generated in a halftone dot image having a fine periodical pattern.
As a method to make interpolation error small in the case of interpolating an image such as a fine halftone dot image having high periodicity, JP-A 2005-141611 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection No.) discloses an interpolating method, in which data for interpolation of a missing pixel is determined such that the average value of a set of plural pixels containing a missing pixel is made to be equal to the average value of a set of plural pixels not containing a missing pixel. In this document, as an example to make those two average values coincide to each other, disclosed is a method in which the total values of the pixels, except a missing pixel, of a set of N pixels containing the missing pixel and the total values of a set of N pixels not containing a missing pixel are determined, and a difference thereof is made to be interpolation data for the missing pixel.
In an interpolation method disclosed in JP-A 2005-141611, since interpolation data for a missing pixel are estimated such that an average value of peripheral pixels not containing the missing pixel is equal to an average values of peripheral pixels containing a missing pixel, reference pixels utilized to determine those average values are limited in the one dimensional direction (the line direction of a contact type line image sensor).
In particular, when two or more missing pixels having an unknown value are included in the range of peripheral pixels containing these missing pixels, intensities of those missing pixels cannot be separately determined only under the condition that an average value of the region containing missing pixels is equal to an average value of the region not containing the missing pixels, thus, the plural missing pixels are only interpolated to have an identical intensity, which means the most appropriate interpolation is impossible. Therefore, in an interpolation method of JP-A 2005-141611, reference pixels to be used to determine the average value are limited in one dimensional direction. That is, suitable interpolation corresponding to periodicity of a two dimensional halftone dot image. Further, when a reference region is widened in the one dimensional direction, reference pixels become distant from a missing pixel, and the relevance to the missing pixel gets smaller.